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multiwalled carbon nanotubes by microemulsion for electrochemical supercapacitor
Materials Research Bulletin 43 (2008) 2818–2824 www.elsevier.com/locate/matresbu
Short communication
Synthesis of Ru/multiwalled carbon nanotubes by microemulsion for electrochemical supercapacitor Shancheng Yan, Peng Qu, Haitao Wang, Tian Tian, Zhongdang Xiao * State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Si Pai Lou 2#, Nanjing 210096, PR China Received 22 June 2007; received in revised form 15 September 2007; accepted 18 October 2007 Available online 20 February 2008
Abstract An efficient way to decorate multiwalled carbon nanotubes with Ru had been developed. In this method, Ru nanoparticles were prepared by water-in-oil reverse microemulsion, and the produced Ru anchored on MWCNTs. Transmission electron microscopy (TEM) result showed that RuO2 nanoparticles had the uniform size distribution after electrochemical oxidation. Energy dispersive X-rays (EDX) spectra elucidated the presence of ruthenium oxide in the as-prepared composites after electrochemical oxidation. Cyclic voltammetry result demonstrated that a specific capacitance of deposited ruthenium oxide electrode was significantly greater than that of the pristine MWCNTs electrode in the same medium. # 2007 Elsevier Ltd. All rights reserved. Keywords: B. Electrochemical properties; B. Chemical synthesis
1. Introduction Supercapacitors are energy storage devices, which exhibit acceptable capacity, high power density and long cycle life [1]. According to energy storage mechanism, there are two types of supercapacitors, viz., double-layer and redox supercapacitors. In the former, energy storage arises mainly from the separation of electronic and ionic charges at the interface between the electrode materials and the electrolyte solution. In the latter, fast faradic reactions take place at the electrode materials [2]. Hence, it is obvious that the charge stored in redox supercapacitors should be higher than that in double-layer counterparts [3]. Electrode materials with electronic conductivity such as carbon black, carbon nanotubes, carbon fiber, etc. [4,5] are generally used as electronic conductors. Many transition metal oxides, such as Ru, Mn, Ni, etc. also have been used as electrode materials [6–8]. It is well known that hydrous RuO2 is an excellent material with a remarkable high specific capacitance value [9]. Recently, the enhanced capacitance of multiwalled CNTs functionalized with ruthenium oxide has been widely researched. In this work, the synthesis of Ru/MWCNTs nanocomposites through microemulsion method was reported. The resulting Ru/MWCNTs composites were characterized by energy dispersive X-rays (EDX) spectra and transmission electron microscopy (TEM). The electrochemical behavior of the composites electrodes was tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).
* Corresponding author. Tel.: +86 25 83790820; fax: +86 25 83795635. E-mail address: [email protected] (Z. Xiao). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.041
S. Yan et al. / Materials Research Bulletin 43 (2008) 2818–2824
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2. Experimental The MWCNTs used in this work were synthesized in our laboratory. RuCl3xH2O used in this work was from Alfa Aesar (USA). All other chemicals were purchased from chemical reagent Co., Ltd. and used as received. Triple-distilled water of 18 M obtained from aqua MAX water system (ASW1-0501-U) was used to prepare all the solutions. The preparation of Ru nanoparticles was by a two-emulsion technique. The microemulsion system used in this study consisted of TritonX-100 as a surfactant, isopropyl alcohol as a cosurfactant, cyclohexane as the continuous oil phase. The details were described by Zhang and Chan [10]. The prepared Ru nanoparticles and MWCNTs were dispersed in anhydrous ethanol by ultrasonic. A drop of this solution was coated on the polished surface of homemade carbon paste electrode (CPE) and dried under room temperature. A similar procedure was followed to prepare the MWCNTs electrode. The Ru nanoparticles in the composite were electrochemically oxidized by sweeping the voltage from open circuit potential to 0.75 V vs. Hg/Hg2Cl2 electrode and holding at 0.75 V for 2 h [11]. The morphologies and microstructures of the products were examined by means of TEM on a Tecnai G2 STWIN transmission electron microscope equipped with an EDX spectrometer using an accelerating voltage of 200 kV. The electrochemical measurements were performed by means of an electrochemical analyzer system, CHI 660 C (CH Instruments). The impedance spectrum analyzer, IM6ex (ZAHNER) was employed to measure and analyze the AC-impedance spectra. The potential aptitude of AC was equal to 10 mV and its frequency ranged from 0.01 Hz to 100 kHz. All experiments were carried out in a three-compartment cell. A Hg/Hg2Cl2 electrode and a piece of platinum were employed as the reference and the counter electrode at room temperature, respectively. 3. Results and discussion The effects of the initial mass ratio of Ru to MWCNTs on the composition of the prepared composites were investigated. Fig. 1 shows the TEM image of MWCNTs. As seen in Fig. 1, multiwalled carbon nanotubes are entangled. The average diameter of MWCNTs is about 10–40 nm. A little amorphous carbon attaching on the MWCNTs can be found. The typical TEM image of resulting Ru/MWCNTs composite is shown in Fig. 2. As seen in Fig. 2a, RuO2 nanoparticles are anchored on the surface of MWCNTs. As seen in Fig. 2b, the RuO2 nanoparticles are uniform in size.
Fig. 1. Transmission electron microscopic image of MWCNTs.
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Fig. 2. Transmission electron microscopic image of 40%Ru/MWCNTs.
The average diameters of RuO2 nanoparticles are 1–3 nm. The facts indicate that the prepared Ru nanoparticles by the microemulsion can be easily attached on the MWCNTs and dispersed in the composites. The resulting Ru/MWCNTs composite is further characterized using EDX to examine their chemical composition. Fig. 3 shows that the composite is composed of elements Ru, O and C. The peak of element Cu is caused by the Cu micro-grid on which the product was loaded. The value 40% of Ru content was the weight ratio in the initial experiment. In fact, the Ru content can be calculated from elemental analysis in TEM EDXA. The Ru content has been calculated to be 35.5%, close to the initial weight ratio of the MWCNT and the RuO2. Combining these results with electron microscopy analysis, it is reasonable to conclude that ruthenium oxide particles had been anchored on the carbon nanotubes to obtain carbon nanocomposites. The electrochemical characteristics of these electrodes were tested through cyclic voltammetry using 1.0 M H2SO4 as an electrolyte, in the voltage range of 0–1.0 Vat various potential scan rates. The results are presented in Fig. 4. Also Table 1 shows the specific capacitances of the four types of electrode. According to the shapes of these curves, it can be
S. Yan et al. / Materials Research Bulletin 43 (2008) 2818–2824
Fig. 3. Energy dispersive X-rays spectra of RuO2/MWCNTs.
Fig. 4. Cyclic voltammograms of four types of electrodes recorded in 1.0 M H2SO4 at a scan rate of: (a) 50 and (b) 100 mV s1.
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Fig. 5. The impedance plots of three types of electrodes (a) CPE, (b) CPE/MWCNTs and (c) CPE/MWCNTs/20%Ru.
assumed that their current responses are sweep-rate dependent. The specific capacitance of all four types of electrode were calculated from the cyclic voltammograms according to the equation: C = (qa + qb)/2mDV, where qa, qb, m and DV are the sums of anodic and cathodic voltammetric charges on the anodic and cathodic scans, the mass of the active material and the potential range of the cyclic voltammogram, respectively [3]. At any scan rate, the specific capacitance is found to follow the order: CPE/MWCNTs/40% Ru > CPE/MWCNTs/20% Ru > CPE/MWCNTs > CPE. It is interesting to note that the RuO2 modified electrode shows a very high capacitance of 457.63 F g1, at 100 mV s1. All four types of electrode are found to show good retention of high-rate capacitance.
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Table 1 Values of specific capacitance measured at various scan rates in CV Scan rate (mV s1)
Electrode
Weight of MWCNT (mg)
Weight of Ru (mg)
Capacitance (F g1)
50
CPE CPE/MWCNTs CPE/MWCNTs/20%Ru CPE/MWCNTs/40%Ru
0 0.042 0.030 0.095
0 0 0.0076 0.063
9.42 73.3 247.57 448.84
100
CPE CPE/MWCNTs CPE/MWCNTs/20%Ru CPE/MWCNTs/40%Ru
0 0.042 0.030 0.095
0 0 0.0076 0.063
13.45 116.2 277.63 457.63
The pronounced enhancement in the capacitance of the modified CNT composites electrode likely results from a pseudocapacitance of RuO2 available for the oxidation and reverse reduction through the following electrochemical protonation [12,13]. RuO2 þ dHþ þ de ! RuO2d ðOHÞd
ð1 d 0Þ
The increment of the specific capacitance can be attributed to the larger content of RuO2 in the as-prepared composites synthesized. Ru/MWCNTs composites, having the combined characteristics of high double-layer capacitance and faradaic pseudocapacitance, could become a potential candidate for use as superior energy density electrochemical capacitors. To complete the characterization of the capacitor performance, the electrochemical impedance was carried out. The typical plots were shown in Fig. 5. The internal impedance involves two mechanisms including resistances of mass transport and electrode materials [14]. In our experiment, the mass-transport resistance of the electrolyte is almost constant. Consequently, this value represents the performance of electrode materials. Two distinct regions are shown, dependent on the frequency range [15,16]. The impedance of these capacitors increases and tends to become purely capacitive (vertical lines characteristic of a limiting diffusion process) in the low frequency region. From the point intersecting with the real axis, the internal resistances of these supercapacitors can be estimated in the high frequency region. The small intersected value at horizontal axis indicates better electronic conductivity of the electrode materials. 4. Conclusions In summary, significant enhancement in specific Ru/MWCNTs capacitance up to 457.63 F g1 at 100 mV s1 has been prepared via microemulsion method. Transmission electron microscopy result showed that Ru nanoparticles had the uniform size distribution. Cyclic voltammetry result demonstrated that a specific capacitance of deposited ruthenium electrode was significantly increased, due to the contribution of Ru. The as-prepared composites may be used as a promising candidate for the supercapacitor materials. Acknowledgements The work was supported by National Natural Science Foundation of China (No. 20305003) and Jiangsu Provincial Natural Science Foundation (No. BK2003407). References [1] [2] [3] [4] [5] [6]
C.C. Hu, W.C. Chen, Electrochim. Acta 49 (2004) 3469–3477. Z. Fan, J.H. Chen, K.Z. Cui, F. Sun, Y. Xu, Y.F. Kuang, Electrochim. Acta 52 (2007) 2959–2965. R.Y. Song, J.H. Park, S.R. Sivakkumar, S.H. Kim, J.M. Ko, D.Y. Park, S.M. Jo, D.Y. Kim, J. Power Sources 166 (2007) 297–301. C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Nano Lett. 6 (2007) 2690–2695. P.L. Taberna, G. Chevallier, P. Simon, D. Plee, T. Aubert, Mater. Res. Bull. 41 (2006) 478–484. T.P. Gujar, V.R. Shinde, C.D. Lokhande, W.Y. Kim, K.D. Jung, O.S. Joo, Electrochem. Commun. 9 (2007) 504–510.
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[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Z. Fan, J.H. Chen, M.Y. Wang, K.Z. Cui, H.H. Zhou, Y.F. Kuang, Diamond Relat. Mater. 15 (2006) 1478–1483. F. Tao, Y.Q. Zhao, G.Q. Zhang, H.L. Li, Electrochem. Commun. 9 (2007) 1282–1287. L.M. Huang, H.Z. Li, T.C. Wen, A. Gopalan, Electrochim. Acta 52 (2006) 1058–1063. X. Zhang, K.Y. Chan, Chem. Mater. 15 (2003) 451–459. J.M. Miller, B. Dunn, Langmuir 15 (1999) 799–806. S. Music, S. Popovic, M. Maljkovic, K. Furic, A. Gajovic, Mater. Lett. 56 (2002) 806–811. J.K. Lee, H.M. Pathan, K.D. Jung, O.S. Joo, J. Power Sources 159 (2006) 1527–1531. W.C. Fang, O. Chyan, C.L. Sun, C.T. Wu, C.P. Chen, K.H. Chen, L.C. Chen, J.H. Huang, Electrochem. Commun. 9 (2007) 239–244. Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (2004) 1957–1962. G. Lota, E. Frackowiak, J.J. Mittal, M. Monthioux, Chem. Phys. Lett. 434 (2007) 73–77.
Short communication
Synthesis of Ru/multiwalled carbon nanotubes by microemulsion for electrochemical supercapacitor Shancheng Yan, Peng Qu, Haitao Wang, Tian Tian, Zhongdang Xiao * State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Si Pai Lou 2#, Nanjing 210096, PR China Received 22 June 2007; received in revised form 15 September 2007; accepted 18 October 2007 Available online 20 February 2008
Abstract An efficient way to decorate multiwalled carbon nanotubes with Ru had been developed. In this method, Ru nanoparticles were prepared by water-in-oil reverse microemulsion, and the produced Ru anchored on MWCNTs. Transmission electron microscopy (TEM) result showed that RuO2 nanoparticles had the uniform size distribution after electrochemical oxidation. Energy dispersive X-rays (EDX) spectra elucidated the presence of ruthenium oxide in the as-prepared composites after electrochemical oxidation. Cyclic voltammetry result demonstrated that a specific capacitance of deposited ruthenium oxide electrode was significantly greater than that of the pristine MWCNTs electrode in the same medium. # 2007 Elsevier Ltd. All rights reserved. Keywords: B. Electrochemical properties; B. Chemical synthesis
1. Introduction Supercapacitors are energy storage devices, which exhibit acceptable capacity, high power density and long cycle life [1]. According to energy storage mechanism, there are two types of supercapacitors, viz., double-layer and redox supercapacitors. In the former, energy storage arises mainly from the separation of electronic and ionic charges at the interface between the electrode materials and the electrolyte solution. In the latter, fast faradic reactions take place at the electrode materials [2]. Hence, it is obvious that the charge stored in redox supercapacitors should be higher than that in double-layer counterparts [3]. Electrode materials with electronic conductivity such as carbon black, carbon nanotubes, carbon fiber, etc. [4,5] are generally used as electronic conductors. Many transition metal oxides, such as Ru, Mn, Ni, etc. also have been used as electrode materials [6–8]. It is well known that hydrous RuO2 is an excellent material with a remarkable high specific capacitance value [9]. Recently, the enhanced capacitance of multiwalled CNTs functionalized with ruthenium oxide has been widely researched. In this work, the synthesis of Ru/MWCNTs nanocomposites through microemulsion method was reported. The resulting Ru/MWCNTs composites were characterized by energy dispersive X-rays (EDX) spectra and transmission electron microscopy (TEM). The electrochemical behavior of the composites electrodes was tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).
* Corresponding author. Tel.: +86 25 83790820; fax: +86 25 83795635. E-mail address: [email protected] (Z. Xiao). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.041
S. Yan et al. / Materials Research Bulletin 43 (2008) 2818–2824
2819
2. Experimental The MWCNTs used in this work were synthesized in our laboratory. RuCl3xH2O used in this work was from Alfa Aesar (USA). All other chemicals were purchased from chemical reagent Co., Ltd. and used as received. Triple-distilled water of 18 M obtained from aqua MAX water system (ASW1-0501-U) was used to prepare all the solutions. The preparation of Ru nanoparticles was by a two-emulsion technique. The microemulsion system used in this study consisted of TritonX-100 as a surfactant, isopropyl alcohol as a cosurfactant, cyclohexane as the continuous oil phase. The details were described by Zhang and Chan [10]. The prepared Ru nanoparticles and MWCNTs were dispersed in anhydrous ethanol by ultrasonic. A drop of this solution was coated on the polished surface of homemade carbon paste electrode (CPE) and dried under room temperature. A similar procedure was followed to prepare the MWCNTs electrode. The Ru nanoparticles in the composite were electrochemically oxidized by sweeping the voltage from open circuit potential to 0.75 V vs. Hg/Hg2Cl2 electrode and holding at 0.75 V for 2 h [11]. The morphologies and microstructures of the products were examined by means of TEM on a Tecnai G2 STWIN transmission electron microscope equipped with an EDX spectrometer using an accelerating voltage of 200 kV. The electrochemical measurements were performed by means of an electrochemical analyzer system, CHI 660 C (CH Instruments). The impedance spectrum analyzer, IM6ex (ZAHNER) was employed to measure and analyze the AC-impedance spectra. The potential aptitude of AC was equal to 10 mV and its frequency ranged from 0.01 Hz to 100 kHz. All experiments were carried out in a three-compartment cell. A Hg/Hg2Cl2 electrode and a piece of platinum were employed as the reference and the counter electrode at room temperature, respectively. 3. Results and discussion The effects of the initial mass ratio of Ru to MWCNTs on the composition of the prepared composites were investigated. Fig. 1 shows the TEM image of MWCNTs. As seen in Fig. 1, multiwalled carbon nanotubes are entangled. The average diameter of MWCNTs is about 10–40 nm. A little amorphous carbon attaching on the MWCNTs can be found. The typical TEM image of resulting Ru/MWCNTs composite is shown in Fig. 2. As seen in Fig. 2a, RuO2 nanoparticles are anchored on the surface of MWCNTs. As seen in Fig. 2b, the RuO2 nanoparticles are uniform in size.
Fig. 1. Transmission electron microscopic image of MWCNTs.
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Fig. 2. Transmission electron microscopic image of 40%Ru/MWCNTs.
The average diameters of RuO2 nanoparticles are 1–3 nm. The facts indicate that the prepared Ru nanoparticles by the microemulsion can be easily attached on the MWCNTs and dispersed in the composites. The resulting Ru/MWCNTs composite is further characterized using EDX to examine their chemical composition. Fig. 3 shows that the composite is composed of elements Ru, O and C. The peak of element Cu is caused by the Cu micro-grid on which the product was loaded. The value 40% of Ru content was the weight ratio in the initial experiment. In fact, the Ru content can be calculated from elemental analysis in TEM EDXA. The Ru content has been calculated to be 35.5%, close to the initial weight ratio of the MWCNT and the RuO2. Combining these results with electron microscopy analysis, it is reasonable to conclude that ruthenium oxide particles had been anchored on the carbon nanotubes to obtain carbon nanocomposites. The electrochemical characteristics of these electrodes were tested through cyclic voltammetry using 1.0 M H2SO4 as an electrolyte, in the voltage range of 0–1.0 Vat various potential scan rates. The results are presented in Fig. 4. Also Table 1 shows the specific capacitances of the four types of electrode. According to the shapes of these curves, it can be
S. Yan et al. / Materials Research Bulletin 43 (2008) 2818–2824
Fig. 3. Energy dispersive X-rays spectra of RuO2/MWCNTs.
Fig. 4. Cyclic voltammograms of four types of electrodes recorded in 1.0 M H2SO4 at a scan rate of: (a) 50 and (b) 100 mV s1.
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Fig. 5. The impedance plots of three types of electrodes (a) CPE, (b) CPE/MWCNTs and (c) CPE/MWCNTs/20%Ru.
assumed that their current responses are sweep-rate dependent. The specific capacitance of all four types of electrode were calculated from the cyclic voltammograms according to the equation: C = (qa + qb)/2mDV, where qa, qb, m and DV are the sums of anodic and cathodic voltammetric charges on the anodic and cathodic scans, the mass of the active material and the potential range of the cyclic voltammogram, respectively [3]. At any scan rate, the specific capacitance is found to follow the order: CPE/MWCNTs/40% Ru > CPE/MWCNTs/20% Ru > CPE/MWCNTs > CPE. It is interesting to note that the RuO2 modified electrode shows a very high capacitance of 457.63 F g1, at 100 mV s1. All four types of electrode are found to show good retention of high-rate capacitance.
S. Yan et al. / Materials Research Bulletin 43 (2008) 2818–2824
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Table 1 Values of specific capacitance measured at various scan rates in CV Scan rate (mV s1)
Electrode
Weight of MWCNT (mg)
Weight of Ru (mg)
Capacitance (F g1)
50
CPE CPE/MWCNTs CPE/MWCNTs/20%Ru CPE/MWCNTs/40%Ru
0 0.042 0.030 0.095
0 0 0.0076 0.063
9.42 73.3 247.57 448.84
100
CPE CPE/MWCNTs CPE/MWCNTs/20%Ru CPE/MWCNTs/40%Ru
0 0.042 0.030 0.095
0 0 0.0076 0.063
13.45 116.2 277.63 457.63
The pronounced enhancement in the capacitance of the modified CNT composites electrode likely results from a pseudocapacitance of RuO2 available for the oxidation and reverse reduction through the following electrochemical protonation [12,13]. RuO2 þ dHþ þ de ! RuO2d ðOHÞd
ð1 d 0Þ
The increment of the specific capacitance can be attributed to the larger content of RuO2 in the as-prepared composites synthesized. Ru/MWCNTs composites, having the combined characteristics of high double-layer capacitance and faradaic pseudocapacitance, could become a potential candidate for use as superior energy density electrochemical capacitors. To complete the characterization of the capacitor performance, the electrochemical impedance was carried out. The typical plots were shown in Fig. 5. The internal impedance involves two mechanisms including resistances of mass transport and electrode materials [14]. In our experiment, the mass-transport resistance of the electrolyte is almost constant. Consequently, this value represents the performance of electrode materials. Two distinct regions are shown, dependent on the frequency range [15,16]. The impedance of these capacitors increases and tends to become purely capacitive (vertical lines characteristic of a limiting diffusion process) in the low frequency region. From the point intersecting with the real axis, the internal resistances of these supercapacitors can be estimated in the high frequency region. The small intersected value at horizontal axis indicates better electronic conductivity of the electrode materials. 4. Conclusions In summary, significant enhancement in specific Ru/MWCNTs capacitance up to 457.63 F g1 at 100 mV s1 has been prepared via microemulsion method. Transmission electron microscopy result showed that Ru nanoparticles had the uniform size distribution. Cyclic voltammetry result demonstrated that a specific capacitance of deposited ruthenium electrode was significantly increased, due to the contribution of Ru. The as-prepared composites may be used as a promising candidate for the supercapacitor materials. Acknowledgements The work was supported by National Natural Science Foundation of China (No. 20305003) and Jiangsu Provincial Natural Science Foundation (No. BK2003407). References [1] [2] [3] [4] [5] [6]
C.C. Hu, W.C. Chen, Electrochim. Acta 49 (2004) 3469–3477. Z. Fan, J.H. Chen, K.Z. Cui, F. Sun, Y. Xu, Y.F. Kuang, Electrochim. Acta 52 (2007) 2959–2965. R.Y. Song, J.H. Park, S.R. Sivakkumar, S.H. Kim, J.M. Ko, D.Y. Park, S.M. Jo, D.Y. Kim, J. Power Sources 166 (2007) 297–301. C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Nano Lett. 6 (2007) 2690–2695. P.L. Taberna, G. Chevallier, P. Simon, D. Plee, T. Aubert, Mater. Res. Bull. 41 (2006) 478–484. T.P. Gujar, V.R. Shinde, C.D. Lokhande, W.Y. Kim, K.D. Jung, O.S. Joo, Electrochem. Commun. 9 (2007) 504–510.
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[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Z. Fan, J.H. Chen, M.Y. Wang, K.Z. Cui, H.H. Zhou, Y.F. Kuang, Diamond Relat. Mater. 15 (2006) 1478–1483. F. Tao, Y.Q. Zhao, G.Q. Zhang, H.L. Li, Electrochem. Commun. 9 (2007) 1282–1287. L.M. Huang, H.Z. Li, T.C. Wen, A. Gopalan, Electrochim. Acta 52 (2006) 1058–1063. X. Zhang, K.Y. Chan, Chem. Mater. 15 (2003) 451–459. J.M. Miller, B. Dunn, Langmuir 15 (1999) 799–806. S. Music, S. Popovic, M. Maljkovic, K. Furic, A. Gajovic, Mater. Lett. 56 (2002) 806–811. J.K. Lee, H.M. Pathan, K.D. Jung, O.S. Joo, J. Power Sources 159 (2006) 1527–1531. W.C. Fang, O. Chyan, C.L. Sun, C.T. Wu, C.P. Chen, K.H. Chen, L.C. Chen, J.H. Huang, Electrochem. Commun. 9 (2007) 239–244. Y.G. Wang, X.G. Zhang, Electrochim. Acta 49 (2004) 1957–1962. G. Lota, E. Frackowiak, J.J. Mittal, M. Monthioux, Chem. Phys. Lett. 434 (2007) 73–77.